using Spotify via Apple Car Play is in principal impossible. Sometimes, the tabs are not shown at all on my car screen - even though my iPhone is unlocked.
Coming to the bigger issue:
when the tabs are shown and I e.g. go to my Albums: the list of albums is shown. But when I click on an album, I get the message that it can't be played (or sometimes "[null] can't be played". (the album has been downloaded already) It never works to select an album and play it via Apple Car Play. I have the same issue with an iPhone X (same iOS version). And this issue not only exists in my car. (tried it also in several other cars)